Synthesis of soybean oil based polyester plasticizer and its property mixing with polyvinyl chloride

Abstract: The utilization of renewable resources in material application attracts increasing attentions in chemical industry, due to the concern regarding environmental sustainability. Nowadays, most commercially available materials are derived from non-renewable resources. With rapid consumption of fossil oils and dramatic fluctuations in the oil price, there is an urgent need to develop chemical materials from renewable resources. Vegetable oils are considered to be important renewable sources due to the rich varieties suitable for chemical transformation, universal availability and low price, and they are the preferred alternatives by chemical industry. In this study, soybean oil monoglycerides were prepared by the transesterification between soybean oil and glycerol at 220~240℃, then polyester plasticizer based on soybean oil was synthesized from soybean oil monoglycerides and maleic anhydride via direct esterification and polycondensation route. The soybean oil polyester plasticizer was characterized by Gel Permeation Chromatography, FT-IR spectrum and HNMR Spectroscopy. The results showed that the molar mass increased obviously with the extension of reaction time. The molar mass of Mn increased from 3027 g/mol to 3030 g/mol when reaction time was 10 h. And the distribution index was about 1.79-2.10. The blends of soybean oil polyester and polyvinyl chloride (PVC) underwent a melting process. The compatibility, mechanical properties and thermodynamic properties of the blends were characterized by torque rheometer, universal testing machine, thermogravimetric analysis (TGA) and dynamic thermo mechanical analysis (DMA). The results showed that with polyester content increasing from 10 g to 30 g in PVC blends, the TG curves showed a three-stage thermal degradation process above 80℃. PVC blends were thermally stable in N2 atmosphere. The first-stage degradation at around 80℃-280℃ could be attributed to the evaporation of water and small molecules. The degradation in the second stage at around 280℃-470℃ was the fastest and corresponding to the formation and stoichiometric elimination of HCl. The last-stage degradation at above 470℃ was attributed to cross linking containing C=C bonds and the degradation of inert filler such as thermal stabilizers. The glass-transition temperature (Tg) decreased from 55℃ to 42℃, and the change of the Tg of PVC blends was in agreement with the common rule the plasticizer can decrease the Tg of PVC. The reason was that the polyester based on soybean oil could interact with PVC molecular and increase the mobility of PVC chain segment. The scanning electron microscope images of the surface of sample-1 plasticized with polyester plasticizer based on soybean oil presented many particles and fractures on the external surface, the particle appearing on the surface was properly the result of some plasticizer excess that was scattered outside the PVC matrix. It caused the eventual migration of plasticizer out of PVC in the processing and using. The SEM images of the surface of sample-3 showed homogeneous and smooth surface. It indicated that the plasticizer was uniformly dispersed in the PVC matrix and surrounded intimately by PVC continuous phase, and there was not a clear boundary between them to be observed. So there was good compatibility between polyester plasticizer based on soybean oil and PVC. The torque of PVC blends decreased from 13.4 N·m to 10.1 N·m, which corresponded to a decrement of 24.6%. The torque decrement could decrease the melt viscosity and would be conductive to thermoplastic processing of PVC blends. The enhancement in plasticization will expand the application range of PVC plasticized with polyester plasticizer materials. This study may lead to the development of new type of PVC plasticizer based on vegetable oil.